The invention relates to a cooling arrangement for cooling a fuel cell in a fuel cell system.
Cooling arrangements for cooling a fuel cell are known from the general prior art. In particular in vehicle applications of fuel cells or fuel cell systems, high-efficiency cooling, on the one hand, and a good option for heating up the fuel cell rapidly in the case of a cold start, on the other hand, play a decisive role.
A fuel cell system having a cooling circuit of the type in question is known in greater detail from DE 102 03 311 B4. For the above-mentioned reasons, this cooling circuit has a controllable radiator bypass, to thus achieve a flow of liquid coolant medium through the cooling heat exchanger in regular operation and to bypass the cooling heat exchanger for the cold start case, to thus be able to heat up the fuel cell more rapidly. This “small” cooling circuit through the radiator bypass comprises, in addition to the fuel cell heat exchanger, a charge air cooler and a coolant pump. In the cold start case, the fuel cell can thus be heated comparatively rapidly using heat from the charge air cooler via the fuel cell heat exchanger. In regular operation, both the charge air cooler and also the fuel cell are then cooled via the fuel cell heat exchanger, wherein the liquid coolant medium heated in the charge air cooler and the fuel cell heat exchanger is cooled via the cooling heat exchanger. The construction of the cooling arrangement described in the prior art of the type in question has two disadvantages in its core concept in this case. On the one hand, the arrangement of the charge air cooler and the fuel cell heat exchanger in the small cooling circuit does result in rapid heating of the fuel cell in the cold start case. On the other hand, the construction described therein, in which flow occurs firstly through the charge air cooler and then the fuel cell has the disadvantage for regular operation, i.e., cooling operation for the fuel cell, that the coolant medium has already been heated relatively strongly via the charge air cooler, before it flows into the fuel cell heat exchanger. This impairs efficient cooling of the fuel cell. A further very severe disadvantage is that the coolant pump has to be designed for the highest load operation of the cooling. It is therefore overdimensioned for the cold start case and the operation of the small cooling circuit and accordingly has worse efficiency in the case of the required volume flows and delivery rates occurring therein. This represents a significant disadvantage, in particular because in these situations all of the power required for heating the fuel cell firstly has to come from an electrical energy store, for example a battery. A coolant pump which is inefficient in this situation therefore results in the necessity of a comparatively large electrical energy storage device, with the corresponding disadvantages with respect to installation space and costs. In particular in an application in a vehicle, this represents a significant disadvantage.
A similar construction having a main cooling circuit divided into multiple branches and a small cooling circuit operated in the cold start case having the fuel cell heat exchanger and a heating unit is known from the further prior art in the form of EP 1 836 742 B1. The construction, as can already be seen from the illustration of the figure, is extraordinarily complex and requires a variety of valve units, which accordingly require a large amount of installation space and cause a need for control and costs. A further disadvantage here is the drive of the coolant pump in the small circuit via an expander and therefore indirectly via the main coolant pump. This construction has a correspondingly poor efficiency, so that here the main coolant pump requires even more power for the case of a cold start than in the above-described exemplary embodiment, so that this arrangement, in addition to the above-mentioned disadvantages, also has the above-described severe disadvantage with respect to the high demand for power in the cold start.
The object of the present invention here is to specify a cooling arrangement for a fuel cell in a fuel cell system, which is improved in relation to the prior art, and which in particular avoids the above-mentioned disadvantages.
In the cooling arrangement according to the invention, it is provided, similarly to the prior art of the type in question, that the cooling arrangement has at least two cooling circuits which are fluidically connected to one another, and in which a liquid coolant medium flows. The first cooling circuit comprises in this case a first coolant pump, a fuel cell heat exchanger, a heating unit, and a charge air cooler. The second cooling circuit comprises a cooling heat exchanger for cooling the coolant medium. It is provided according to the invention in this case that an additional coolant pump is provided. The construction can now be operated so that, for example, for the cold start case, essentially the first cooling circuit having its coolant pump, the fuel cell heat exchanger, the charge air cooler, and the heating unit is operated. Heat is introduced into the liquid coolant medium in the first cooling circuit by the heating unit and, as soon as the air supplied to the cathode side of the fuel cell is started, by the supply air, which is hot after the compression. Since the first cooling circuit can be operated independently of the second cooling circuit, efficient heating of the fuel cell via the heat from the compressed supply air and the heat introduced via the heating unit, for example, an electrical resistance heater, can thus be performed with correspondingly lower thermal mass. Because the first cooling circuit can be operated independently of the second cooling circuit, cooling of the liquid coolant medium in the cooling heat exchanger is prevented, so that the construction accordingly heats up rapidly.
In contrast to a solution having a radiator bypass around the cooling heat exchanger, as is known from the prior art, this construction has the decisive advantage in this case that comparatively efficient circulation of the liquid coolant medium in the first cooling circuit can be performed by the second coolant pump, which typically does not have to be operated in this situation of heating the fuel cell. This is because the first coolant pump can be optimized for a lower volume flow and a lower delivery rate than the pumps in the prior art. Therefore, with ideal efficiency of the first coolant pump for the described intended use, very efficient circulation of the liquid coolant medium for heating the fuel cell can be achieved. This represents a substantial advantage with respect to the capacity and the construction of the electrical energy storage unit, in particular in an application in a vehicle, in which the electrical power required for this purpose for the coolant pump, at least in the starting phase, originates from an electrical energy storage unit.
In a regular operating situation, in which the fuel cell has to be cooled, for example, in the case of cooling the fuel cell in full-load operation of a vehicle equipped with the fuel cell system, both coolant pumps can then assume the delivery of the liquid coolant medium in series one behind the other. In this way, a greater volume flow is accordingly possible, wherein the individual coolant pumps can nonetheless be optimized as small and efficient for the respective operating situation, in which they primarily have to assume the load of the delivery of the volume flow of liquid coolant medium. Thus, for example, two small and therefore cost-effective pumps may be used, which together supply the required maximum rate, and if they are each operated alone, can provide a smaller volume flow at high efficiency.
A further very advantageous embodiment of the cooling arrangement according to the invention provides in this case that the charge air cooler is arranged after the fuel cell heat exchanger in the flow direction of the coolant medium. This arrangement of the charge air cooler in the flow direction of the coolant medium after the fuel cell heat exchanger ensures in regular operation that the coolant medium, before it enters the fuel cell, accordingly has not already been strongly heated by the charge air cooler, which would represent a not insignificant disadvantage. Rather, via the described arrangement, the fuel cell can have incident flow of comparatively cool coolant medium and can be cooled reliably and efficiently. The temperature of the coolant medium when it leaves the fuel cell or the fuel cell heat exchanger again is still sufficient in this case to cool down the supply air, which is comparatively hot after the compression, to an extent which is necessary for the reliable operation of the fuel cell. The coolant medium will then have a correspondingly high temperature after the charge air cooler. Ideally, it then flows more or less directly to the cooling heat exchanger in cooling operation. Therein, the possible cooling is typically dependent on the temperature difference between the ambient air and the liquid coolant medium flowing into the cooling heat exchanger. The higher this temperature difference, the higher the temperature of the liquid coolant medium at this point of the cooling arrangement—with ambient temperature which is typically not to be influenced—the better and more efficient the cooling. Due to the arrangement of the charge air cooler in the flow direction of the coolant medium, ideally directly before the cooling heat exchanger, particularly efficient cooling of the liquid coolant medium is thus achieved. Accordingly, the surface area of the cooling heat exchanger can be reduced or a higher cooling power can be achieved with equal surface area. This is of decisive advantage in particular in vehicle operations, because the surface area available here of the cooling heat exchanger is typically due to design specifications, which represents a severe disadvantage in regular cooling arrangements for fuel cells, because of the comparatively low operating temperature of a fuel cell in comparison to an internal combustion engine. This can be significantly reduced by the described arrangement of the charge air cooler in the flow direction after the fuel cell heat exchanger.
In heating operation, in which the fuel cell is to be heated, in the cooling arrangement according to the invention, only the first cooling circuit has a comparatively small thermal mass flowing through it. In this case, it plays approximately no role whether the charge air cooler is arranged before or after the fuel cell in the flow direction of the coolant medium. Because of the comparatively short running length of the liquid coolant medium in the first cooling circuit, the heat from the charge air cooler is moved comparatively efficiently into the fuel cell heat exchanger after flowing through the entire first circuit even in the above-described arrangement. Because of the above-described advantages in regular operation, the arrangement according to this embodiment is therefore preferable in any case. A further advantage of the arrangement of the charge air cooler and the fuel cell heat exchanger in the same (small) cooling circuit is additionally that the air for the cathode side of the fuel cell and the coolant medium for the fuel cell heat exchanger are temperature-controlled uniformly by the charge air cooler. Good water management may be achieved in the fuel cell in this way. Due to the uniform heating of the fuel cell itself via the fuel cell heat exchanger and via the air flowing to the cathode side of the fuel cell from the charge air cooler, in addition to good water management, the introduction of thermal tensions into the region of the fuel cell is also reduced, which results in a longer service life of the fuel cell, because clamping units and seals of the fuel cell stack are loaded much less than if the fuel cell itself and the air flowing therein have greater temperature differences.
A further very advantageous embodiment of the cooling arrangement according to the invention furthermore provides a chamber heat exchanger for at least indirect heating of a chamber in one of the cooling circuits. Such a chamber heat exchanger is used for heating a chamber arranged in the region of the fuel cell system. In the case of the preferred application in a vehicle, it can be in particular an interior heat exchanger, via which a passenger compartment can be heated. According to a very advantageous refinement of this concept, the chamber heat exchanger is provided in this case in the first cooling circuit. The chamber heat exchanger can thus be provided in a particularly advantageous manner together with the fuel cell, the heating unit, and the charge air cooler and the first coolant pump in the first cooling circuit. In this way, it is possible to perform heating of the chamber, in particular the interior of a vehicle, via the chamber heat exchanger already very early after the start of the vehicle, to thus enable a high level of thermal comfort for the user of the vehicle.
A further very advantageous embodiment of the concept furthermore provides that the second cooling circuit has two partial circuits, which both comprise a coolant pump, wherein the chamber heat exchanger and the heating unit are arranged in the same partial circuit, and wherein the fuel cell heat exchanger is arranged in the other partial circuit. Due to this construction having an allocation of the first cooling circuit into two partial circuits it is possible to operate these partial circuits, each of which has a coolant pump, independently of one another. Thus, a high level of comfort for the chamber, in particular the interior of a vehicle, can be provided. This is because heating of the chamber can be performed via operation of only one partial circuit, which has a coolant pump, the chamber heat exchanger, and the heating unit, without the fuel cell or the fuel cell system also having to be heated or started for this purpose. On the other hand, an operation of the second partial circuit also enables heating of only the fuel cell, without the interior also being heated. This would also be conceivable by not having chamber air flowing through the chamber heat exchanger, but in this case the additional flow resistance of the chamber heat exchanger would be present in the small cooling circuit, so that due to the alternative construction having the partial circuits, a further optimization of the energy required for heating the fuel cell via the first cooling circuit can be achieved.
According to a further advantageous embodiment of the cooling arrangement according to the invention, it can also be provided that the chamber heat exchanger is arranged in a radiator bypass line to the cooling heat exchanger. The chamber heat exchanger can thus be arranged to have flow through it alternatively or additionally in parallel to the cooling heat exchanger and thus, for the case in which the chamber is to be heated, can also assist the cooling of the coolant medium, in particular in regular operation of the fuel cell system. At the same time, components can be saved, since due to the arrangement in the radiator bypass line, no additional line branch is necessary for the chamber heat exchanger with comparable functionality.
A further very advantageous embodiment of the cooling arrangement according to the invention can additionally have at least one heating heat exchanger for heating a component assigned thereto in the flow direction before the fuel cell heat exchanger. Such a heating heat exchanger can thus be used for heating a further component or a further medium. Cooling of the liquid coolant medium in the cooling circuit is thus achieved via the heating heat exchanger. The heating heat exchanger can be used in the fuel cell system, for example, to heat expanded fuel for the fuel cell. For example, if hydrogen from a compressed gas accumulator at a nominal pressure of 70 MPa is used, it typically has to be expanded to a very much lower pressure level, before it is introduced into the fuel cell. The hydrogen cools down very strongly upon this expansion to a lower pressure level. This could result in thermal tensions and possibly in condensing out—and at worst freezing—of moisture in the fuel cell. Therefore, the expanded hydrogen is typically heated. This can now be performed via a heating heat exchanger, which is arranged in the cooling arrangement according to the invention in the described advantageous embodiment before the fuel cell heat exchanger in the flow direction. In this way, the temperature of the liquid coolant medium can be reduced further before the fuel cell heat exchanger, to thus ensure particularly efficient cooling of the fuel cell in regular operation.
The heating heat exchanger can advantageously be provided in the second cooling circuit after the cooling heat exchanger in the flow direction. Alternatively or additionally to heating the expanded hydrogen, it can also be used for heating a water separator, for example a water separator in the cathode exhaust air of the fuel cell, to prevent freezing thereof here. Via both functionalities, which can be arranged alternatively to one another or also in series to one another, preferably with a flow first through the heating heat exchanger for the expanded fuel and then the heating heat exchanger for the water separator, in any case the temperature of the liquid coolant medium is reduced, which results in an advantage in the cooling of the fuel cell.
In this case, according to an advantageous refinement, the heating heat exchanger or exchangers can have the liquid coolant medium flow through them in series or in parallel with an ion exchanger. The temperature is also reduced in the region of an ion exchanger. Moreover, the service life of the ion exchanger may be lengthened by a comparatively low temperature of the liquid coolant medium. Both the ion exchanger and also the heating heat exchanger or exchangers can be arranged in parallel to one of the coolant pumps in this case according to an advantageous refinement of the concept. In this way, the coolant medium flows through sufficiently in cooling operation for the fuel cell, without the entire volume flow of the liquid coolant medium having to flow through the components, which is not necessary, on the one hand, and would result in higher pressure losses and accordingly a poor energetic efficiency, on the other hand.
Further advantageous embodiments of the cooling arrangement according to the invention furthermore result from the further dependent subclaims and will become clear on the basis of the exemplary embodiments, which are described in greater detail hereafter with reference to the figures.